1. Field of the Invention
The present invention is directed to methods and systems for monitoring the integrity of structures and, in particular, to a method and system for monitoring structural reinforcement or securing members, such as post-tensioning cable, in concrete. More particularly still, breakage of such cables is detected, distinguished from background noise, and located by means of subsequent analysis or on a real-time basis. The source of the breakage is located by known triangulation techniques. In addition, cost saving space multiplexing of sensors is used.
2. Related Art
U.S. Pat. No. 3,949,353, granted Apr. 6, 1976 to Waters et al, and titled "Underground Mine Surveillance System", discloses a system for maintaining a continuous log of activity in and around an underground mine where seismic energy in the area is continually monitored, processed and classified into meaningful relative data indications; and which system further includes selectively deployable seismic energy monitoring equipment providing more specific data in the event of mine catastrophe. This system utilizes permanently disposed seismic energy detectors and/or emergency detectors placed in accordance with the particular exigency, and detected seismic energy return is continually processed to maintain a data log indicative of general type and location of mine activity, with particular capability for isolation of unusual seismic events by comparison with statistical data constraints of predetermined character.
U.S. Pat. No. 4,386,343, granted May 31, 1983 to Shiveley, and titled "Acoustic Emission Intruder Alarm System", discloses an acoustic emission burglary detection system for detecting physical attacks made on a protected structure such as a vault, safe or the like. Sensors (13, 14) mounted on the protected structure detect acoustic emission stress wave signals produced by an attack and provide an event signal of a corresponding frequency and with an amplitude and duration dependent upon those of the stress wave signals. Event signals exhibiting a frequency less than 50,000 Hz are much less likely to have been originated by a physical attack upon the protected structure and are filtered out. The remaining event signals which exceed a predetermined level, are integrated over a predetermined time period. If the resulting value exceeds a predetermined level, an alarm is activated. Means are provided for testing the detection circuit by providing electrical pulses through one of the sensors to cause it to generate mechanical stress wave signals in the protected structure which can be detected and processed by the detection circuitry.
U.S. Pat. No. 4,649,524, granted Mar. 10, 1987 to Vance, and titled "Integrated Acoustic Network" discloses an integrated acoustic network system to provide warning of impending groundfall in underground mines. The system includes a plurality of geophones which derive acoustic signals by which the source of seismic disturbances is located, and an array of high frequency piezoelectric sensors which pick up signals from small ground disturbances which precede groundfall. A warning system is provided both at the scene of mining operations and at a central location of impending groundfall and of the location of its occurrence.
In U.S. Pat. No. 3,949,353, unusual seismic events are isolated by comparison with stored statistical data constraints. In U.S. Pat. No. 4,386,343, event signals exhibiting a frequency above 50,000 Hz are integrated over a predetermined time period and, if the resulting value exceeds a predetermined level, an alarm is activated. In U.S. Pat. No. 4,649,524, the number of seismic events is counted and the cumulative amount of energy is estimated for every minute and the ratio of energy/event is calculated.
U.S. Pat. No. 4,535,629, granted Aug. 20, 1985 to Prine, and titled "Method and Apparatus for Structural Monitoring with Acoustic Emission and Using Pattern Recognition", discloses an acoustic emission monitoring system used for monitoring fatigue crack growth in metal or other materials such as occur, for example, in highway bridges during normal traffic loading. The transducers are placed on the plates to be tested to allow detection of acoustic emission from a particular site. By applying specific recognition methods to the acoustic emission AE, detection of flaws can be detected from a random noise background. The pattern recognition technique first subjects the received AE energy to an energy window test and if the energy is within the window, it is subjected to a rate test and if the energy exceeds predetermined rates, it is passed to a location test so as to locate the position of flaws.
U.S. Pat. No. 4,565,964, granted Jan. 21, 1986 to Matthews, et al., and titled "Cable Integrity by Acoustic Emission", discloses a system for monitoring the integrity of a cable, for example a cable following a variable depth sonar body through the ocean. Such a cable comprises a core of electrical wires surrounded by load bearing wires which are secured to the sonar body through a terminator. For various reasons cracks can appear in the load bearing wires and in extreme cases one or more of the wires may break. The monitoring system includes a transducer located near the terminator where the wires are most likely to crack or break. Acoustic emissions caused by the incidence of cracks or breaks are picked up by the transducer. The resulting electrical signals are amplified and passed up the electrical core of the cable to the towing vessel where they are processed. Novel aspects of the system are the water coupling between the location of the cracks or breaks and the transducer and special processing circuitry which enables breaks, cracks and electrical noise to be distinguished from each other.
U.S. Pat. No. 4,738,137, granted Apr. 19, 1988 to Sugg, et al., and titled "Acoustic Emission Frequency Discrimination", discloses an acoustic emission signal processor that selectively sorts acoustic signals on the basis of frequency content, rather than just the frequency. The processor allows rejection of some signals having a particular frequency content, or can provide for separate counting or other processing of these signals. The above mentioned United States patents are incorporated herein by reference.